1. Field of the Invention
This invention relates to laser gyroscopes and, more particularly, to a stimulated Brillouin scattering ring laser gyroscope utilizing a loop of single-mode optical fiber waveguide as the laser cavity. Stimulated Brillouin scattering is induced and utilized as the counter-directionally travelling radiation between which the beat frequency is detected and measured as a function of the angular rate of rotation of the ring laser gyroscope.
2. The Prior Art
Conventional laser gyroscopes utilize the properties of the optical oscillator (laser) and the theory of relativity to produce an integrating rate gyroscope. The laser gyroscope operates on a well-known principle that rotation of an operating ring laser (optical oscillator) about its axis causes the laser cavity to experience an apparent change in length for each direction. This apparent change in length creates a frequency shift in the laser oscillator in each direction. As between two counter-directionally travelling laser oscillations, portions of each may be superimposed so that the frequency shift will be manifested as a beat frequency. This beat frequency is proportional to the rate of angular rotation of the gyroscope and is, therefore, meansurable to provide an indication of the rate of angular rotation of the area circumscribed by the ring laser oscillator or ring laser gyroscope.
The relationship between the observed beat frequency, .DELTA.f, and the rotation rate, .omega., is: EQU .DELTA.f=4A.omega./.lambda.L (1)
where .lambda. is the wavelength of the laser radiation, A is the area enclosed or circumscribed by the ring laser and L is the length of the oscillator cavity.
The fundamental condition is that the laser wavelength, .lambda., in each direction, multiplied by an integer, N, must be equal to the optical path length for the oscillator. This integer, N, is typically in the range of 10.sup.5 to 10.sup.7 (or larger, on certain geophysical applications). EQU L=N.lambda. (2)
accordingly, a change in length, .DELTA.L, will, correspondingly, cause a wavelength change, .DELTA..lambda., as follows: EQU .DELTA..lambda.=.DELTA.L/N (3)
The corresponding frequency change, .DELTA.f, is given as EQU .DELTA.f/f=.DELTA.L/L (4)
therefore, given small length differences, .DELTA.L, and reasonable cavity lengths, L, the operating frequency for a conventional ring laser gyroscope should be as high as possible.
The frequency at which each oscillator operates is determined by the optical path length encountered by the laser radiation in the cavity in which it travels. While apparent path length differences in the conventional ring laser gyroscope (wherein two oscillator paths are contained in essentially the same laser cavity and which length differences are caused by rotation of the single cavity) create a shift in the frequencies in each of the two oscillators, whereas physical changes in cavity length caused by temperature changes, vibration, etc., do not cause frequency differences.
The relationship between inertial input rates, .omega., and apparent length change, .DELTA.L, has been given as EQU .DELTA.L=4A.omega./c (5)
The relationship between .DELTA.f and .omega., in terms of the gyroscope size and length is determined by substituting Equation 4 into Equation 5, giving EQU f=4A.omega./.lambda.L (6)
where c=.lambda.f.
From the foregoing relationship, (Equation 6), it is readily observable that at extremely small angular rotation rates, .omega., the beat frequency, .DELTA.f, for that particular rotation rate, .omega., will also be relatively small.
Thus, the conventional laser gyroscope measures path differences of less than 10.sup.-6 Angstroms, and frequency changes to less than 0.1 Hertz, hereinafter, Hz, (a precision of better than one part in 10.sup.15) in order to read rotation rates of less than 0.1 degrees per hour.
Unfortunately, there is a tendency for a solid state ring laser gyroscope to oscillate in one direction only. This problem is partially alleviated by using a gaseous gain section in the ring laser gyroscope. The gaseous gain section has an inhomogeneous line broadening and can, therefore, oscillate in both directions simultaneously. The electrical discharge in the gaseous gain section, however, introduces turbulence and gas flow with an associated effect referred to as the Fizeau effect. The Fizeau effect is the tendency for the gaseous flow to drag the laser radiation in the direction of the gaseous flow. This introduces certain errors in the accuracy of the ring laser gyroscope.
Additionally, the gas flow in the discharge is almost always turbulent so that bias created by the generally unidirectional gas flow is random thereby creating an error which is difficult to eliminate. One technique to reduce this error has been to produce electrical discharges in two equal-length discharge gaps with equal but oppositely directed electrical currents.
One such ring laser gyroscope is disclosed in U.S. Pat. No. 3,484,169 but has certain inherent limitations to its accuracy by the nature of its construction and components. For example, it uses a gaseous gain section thereby overcoming the serious problem of unidirectional oscillation. However, the electrical discharge in the gaseous medium creates the aforementioned unidirectional gas flow with the associated Fizeau effect so that the gaseous laser cavity is optically longer in one direction than the other even when the ring laser is not rotated.
Additionally, some other limitations of the conventional ring laser gyroscope are phenomena known as "mode pulling" and "lock-in". These phenomena are experienced when the frequency difference between the two oscillators becomes small (less than about 500 Hz). The band width of each oscillator is not sharply defined so that an overlapping of frequencies occurs causing an optical coupling between the two oscillators.
Optical coupling between the two oscillators at low rates of angular rotation is also known to result from scattering caused by imperfections in the single-mode optical fiber waveguide. These imperfections arise from minute flaws such as compositional fluctuations and phase separations in the molten material from which the single-mode optical fiber waveguide is drawn during manufacture. These flaws manifest themselves as elongated imperfections generally parallel to the axis of the single-mode optical fiber waveguide and cause scattering of the oscillator radiation to thereby reduce the delineation of the band width of the optical oscillator. This phenomenon is known in the art as Rayleigh scattering which appears in both the cladding (where it can be absorbed) and in the core as a backward-scattered guided wave.
Another scattering phenomenon which has been observed in optical fiber waveguides is known as Mie scattering. Mie scattering is predominantly a forward scatter and is caused by inhomogeneities comparable in size to the wavelength.
The foregoing scattering and optical coupling between two oscillators operating in essentially the same physical cavity pulls the frequencies closer together (mode pulling) and ultimately locks them together (lock-in) into one frequency, thereby eliminating any beat frequency (dead band) at low frequency differences. Accordingly, when the output of the ring laser oscillator is observed as a function of the rotation rate it is readily seen that, as the rotation rate decreases, the beat frequency rapidly falls to zero before the rotation rate falls to zero as a result of the foregoing phenomena of "lock-in".
Several techniques have been used to reduce the width of this "dead band" and increase the accuracy of the conventional ring laser gyroscope. These techniques include: (1) biasing the ring laser gyroscope by physically increasing the rate of rotation of the laser gyroscope (with a sinusoidally varying angular velocity, for example) and then subtracting out the biasing; or (2) introducing an optical element into the oscillator cavity, the optical element having an index of refraction dependent upon the direction of the laser radiation passing through the element. One of these latter phenomena is known as the Faraday Effect. However, these techniques introduce errors into the system and are also temperature dependent thereby greatly restricting the accuracy and the application capability of the conventional single cavity ring laser gyroscope.
A useful discussion of some of the basic theories involved in the laser gyroscope may be found in IEEE SPECTRUM "The Laser Gyro", Joseph Killpatrick, October, pages 44-55 (1967).
In view of the foregoing, what is needed is an improved laser gyroscope in which the tendency to unidirectionally oscillate and the mode pulling and lock-in phenomena experienced in conventional laser gyroscopes at low rates of angular rotation are significantly reduced. It would also be an improvement in the art to provide a stimulated Brillouin scattering radiation ring laser gyroscope. Another improvement would be to provide a ring laser gyroscope and method whereby the pumping power is significantly reduced. Such an improvement is disclosed in the present invention.